What is the Freshwater Habitat?

Physical Properties of Water:

All freshwater habitats are dominated by the physical properties
of water. The molecule is made up of a single oxygen atom with
two hydrogen atoms attached. The former is slightly negatively
charged whilst the latter is slightly positive. As a consequence,
unlike any other solvent on earth, water can attract, dissolve
and hold mineral ions in solution. A perfect way of providing
nutrients to plants.

Water is viscous and this will produce resistance
to animals moving through it whether this is a tiny insect larva,
fish or an otter. Hence there may be a need to improve streamlining.
Moving water will transport the organism and so those that want
to remain in the same area will need to avoid the faster currents
(see Invertebrate Drift). Moving water creates drag on plants
trying to anchor themselves. If the current is especially fast
silt and mud will be removed and there will be little substrate
in which roots can grow. In fact plants may not be evident and
a closer inspection (including scraping the rock and placing the
debris under the microscope) is needed to see algae and non-flowing
plants attached.

The range of temperatures found in the majority
of habitats is such that water rarely is found in anything other
than liquid. In fact water has a high thermal capacity, meaning
that it needs a great deal of energy to heat it but also retains
it. This means that living in water creates a stable environment.
This stability lasts for much of the year although with the on-set
of winter the drop in temperature causes the water to increase
its density (as in all liquids). Unusually, water has a useful
property in that it is at its most dense at 4 degrees Celcius.
A continuing drop in temperature actually decreases the density
so that as the water freezes it floats above the denser liquid
water. For organisms living in ponds by living at the bottom this
may help them survive the winter. Flowing water requires a much
lower temperatureA stream frozen from the edge with the ice on top. The fastest water is still flowing as it has not frozen

Different Types of Freshwater

Essentially there are two main types, Static Water
(called lentic) and Flowing Water (called lotic). However, this is still vague. For example,
static water could be any size, a puddle left over from the last rain shower
up to Lake Baikal (photo below) in Russia, the deepest freshwater
lake in the world at 1620 metres deep. It also holds the greatest
volume of water in the world with over 300 streams feeding it.
Incidentally Lake Superior in North America has the greatest surface area.

Lake Baikal (photo Vladstudio)

Like lentic systems, flowing water is also varied, from a tiny hill stream bubbling along the edge of a peat bog to a navigable river like the Rhine in Germany. The largest (greatest volume) river system in the world and the
second longest is the River Amazon in South America (the Nile
is the longest at 6741 kilometres). A few stats: it is navigable
for 3700 ks upstream and is 6440 ks at its longest. However, the
drainage basin is almost 6 million square kilometres.

Niagra Falls in winter: located between USA and Canada. These are falls on the Canadian side and the mist of water generated freezes on the surrounding vegetation. The water here is cutting back the land at the rate of nearly 1.5 metres a year

Iguassu Falls: located between Brazil and Argentina. The volume of water flowing over the main cataract (the Devil's Throat, 72 metres high) could fill St Paul's Cathedral in a fraction over 0.5 second. The dark marks in the middle of the spray are swifts that specialise in nesting behind the falls and are flying through it!

Non-natural water Bodies

These are water created by humans. Typically they will fit within the static water section but some may not be obvious. For example where does a canal fit in? Dug out and created as a transport system several hundred years ago they may now be use for amenities such as narrow boat holidays, canoeing or nature reserves for the wildlife. They need careful management as the process of succession will quickly choke them with vegetation. This is because they fit more to the definition of a pond than a river.

A managed canal

What may appear to be a canal could be a drainage channel in a wetland area or on Dartmoor a leat. These are ways of drawing off water for use in industry or drinking. Either way they are channelled and can be fast flowing like an upland river; quite different to the canal.

Reservoirs are often created by damming the head of a valley and allowing water to collect from the natural process of run-off . They are deep and have more in keeping with a lake. They may be used for amenities, i.e. water sports, but may also be stocked with trout for anglers. Stocking may be the only way of ensuring enough fish as there will usually be insufficient food and unsuitable abiotics for breeding.

Acid Lake in Norway

Before the latter part of the twentieth century the lakes in Scandinavia were rich in aquatic life including vegetation and a diversity of animals. The invertebrates provided food for trout and salmon but by the 1980's and 1990's this started to become a rarity. In the recent decades sulphur dioxide gases produced by industries burning fossil fuels has been released into the atmosphere. Primarily this has come from countries like the UK. This acidic gas dissolves in the clouds to fall, many miles from where it was produced, as acid rain. This greatly increases the acidity of the lake killing many of the species living there. This is coupled with the release of aluminium from the soil as the acid rain percolates through and drains to the lakes.